Luca Zangrilli

KPol: liquid crystal polarimeter for K-corona observations from the SCORE coronagraph

We describe the design and first calibration tests of an imaging polarimeter based on Liquid Crystal Variable Retarders (LCVRs), for the study of the solar K-corona. This K-polarimeter (KPol) is part of the visible light path of the UltraViolet and Visible-light Coronal Imager (UVCI) of the Sounding-rocket Coronagraphic Experiment (SCORE). SCORE/UVCI is an externally occulted, off-axis Gregorian telescope, optimized for the narrow-band (i.e., λ/▵λ ~10) imaging of the HeII, λ 30.4 nm and HI λ 121.6 nm coronal emission. We present some preliminary results of the application of LCVR plates to measurements of linear polarized radiation. LCVR plates replace mechanically rotating retarders with electro-optical devices, without no moving parts. LCVR are variable waveplates, in which the change of the retardance is induced by a variable applied voltage. The retardance of a LCVR is a function of the wavelength. KPol observations of the visible coronal continuum of the Sun (K-corona) will be made over the 450-600 nm wavelength band. We have studied the LCVRs properties in this bandpass. We tested a LCVR plate assembled in a linear polarization rotator configuration to measure the polarization plane rotation of input radiation as a function of wavelength. We estimated the LCVRs chromatic response in the KPol wavelength bandpass. The preliminary results show reasonable achromatic behaviour at high regimes of the driving voltage, Vd (i.e., Vd>3 volt).

Ultraviolet and Visible-light Coronagraphic Imager (UVCI)

The HERSCHEL (HElium Resonance Scattering in the Corona and HELiosphere) Sun-Earth Sub-Orbital Program is a proposed sounding-rocket payload designed to investigate helium coronal abundance and solar wind acceleration from a range of solar source structures by obtaining simultaneous observations of the electron, proton and helium solar coronae. HERSCHEL will provide the first measurements of the coronal helium abundance in source regions of the solar wind, thus bringing key elements to our understanding of the Sun-Earth connections. The HERSCHEL instrument package consists of the Extreme Ultraviolet Imaging Telescope (EIT) for on-disk coronal observations and the Ultraviolet and Visible-light Coronagraphic Imager (UVCI) for off-limb observations of the corona. The UVCI is an externally occulted, reflecting coronagraph with an off-axis Gregorian telescope. UVCI will be able to take coronal images at heliocentric heights comprised between 1.2 to 3.5 solar radii of a) K-corona polarized brightness (pB); b) H I Lyman-α, 121.6 nm, line-emission; c) He II Lyman-α, 30.4 nm, line. The key element in the UVCI instrument concept is that the mirrors with multilayer coatings optimized for 30.4 nm still have good reflectivity at 121.6 nm and in the visible. The optical design concept for the UVCI instrument will be discussed, together with its expected optical and throughput performances.

Preliminary results on the characterization and performances of ZBLAN fiber for infrared spectrographs

Present telescopes and future extremely large telescopes make use of fiber-fed spectrographs to observe at optical and infrared wavelengths. The use of fibers largely simplifies the interfacing of the spectrograph to the telescope. At a high spectral resolution (R>50,000) the fibers can be used to achieve very high spectral accuracy. GIANO is an infrared (0.95-2.5μm) high resolution (R=50,000) spectrometer[1] [2] [3] that was recently commissioned at the TNG telescope (La Palma). This instrument was designed and built for direct feeding from the telescope [4]. However, due to constraints imposed on the telescope interfacing during the pre-commissioning phase, it had to be positioned on the rotating building, far from the telescope focus. Therefore, a new interface to the telescope, based on IR-transmitting ZBLAN fibers with 85μm core, was developed. In this article we report the first, preliminary results of the effects of these fibers on the quality of the recorded spectra with GIANO and with a similar spectrograph that we set-up in the laboratory. The effects can be primarily associated to modal-noise (MN) that, in GIANO, is much more evident than in optical spectrometers, because of the much longer wavelengths.

Electro-optical polarimeters for ground-based and space-based observations of the solar K-corona

Polarimeters based on electro-optically tunable liquid crystals (LC) represent a new technology in the field of observational astrophysics. LC-based polarimeters are good candidates for replacing mechanically rotating polarimeters in most ground-based and space-based applications. During the 2006 total solar eclipse, we measured the visible-light polarized brightness (pB) of the solar K-corona with a LC-based polarimeter and imager (E-KPol). In this presentation, we describe the results obtained with the E-KPol, and we evaluate its performances in view of using a similar device for the pB imaging of the K-corona from space-based coronagraphs. Specifically, a broad-band LC polarimeter is planned for the METIS (Multi Element Telescope for Imaging and Spectroscopy) coronagraph for the Solar Orbiter mission to be launched in 2017. The METIS science driver of deriving the coronal electron density from pB images requires an accuracy of better than 1% in the measurement of linear polarization. We present the implications of this requirement on the METIS design to minimize the instrumental polarization of the broad-band visible-light (590-650 nm) polarimeter and of the other optics in the METIS visible-light path. Finally, we report preliminary ellipsometric measurements of the optical components of the METIS visible-light path.

OPSys: optical payload systems facility for testing space coronagraphs

The Turin Astronomical Observatory, Italy, has implemented in ALTEC, Turin, a new Optical Payload Systems (OPSys) facility for testing of contamination sensitive optical space flight instrumentation. The facility is specially tailored for tests on solar instruments like coronagraphs. OPSys comprises an ISO 7 clean room for instrument assembly and a relatively large (4.4 m3) optical test and calibration vacuum chamber: the Space Optics Calibration Chamber (SPOCC). SPOCC consists of a test section with a vacuum-compatible motorized optical bench, and of a pipeline section with a sun simulator at the opposite end of the optical bench hosting the instrumentation under tests. The solar simulator is an off-axis parabolic mirror collimating the light from the source with the solar angular divergence. After vacuum conditioning, the chamber will operate at an ultimate pressure of 10-6 mbar. This work describes the SPOCCs vacuum system and optical design, and the post-flight stray-light tests to be carried out on the Sounding-rocket Experiment (SCORE). This sub-orbital solar coronagraph is the prototype of the METIS coronagraph for the ESA Solar Orbital mission whose closest perihelion is one-third of the Sun-Earth distance. The plans are outlined for testing METIS in the SPOCC simulating the observing conditions from the Solar Orbiter perihelion.

METIS, the Multi Element Telescope for Imaging and Spectroscopy: An instrument proposed for the solar orbiter mission

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is an instrument proposed to the European Space Agency to be part of the payload of the Solar Orbiter mission. The instrument design has been conceived for performing extreme ultraviolet (EUV) spectroscopy both on the solar disk and off-limb, and near-Sun coronagraphy and spectroscopy. The proposed instrument suite consists of three different interconnected elements, COR, EUS and SOCS, sharing the same optical bench, electronics, and S/C heat shield aperture. COR is a visible-EUV multiband coronagraph based on a classical externally occulted design. EUS is the component of the METIS EUV disk spectrometer which includes the telescope and all the related mechanisms. Finally, SOCS is the METIS spectroscopic component including the dispersive system and the detectors. The capability of inserting a small telescope collecting coronal light has been added to perform also EUV coronal spectroscopy. METIS can simultaneously image the visible and ultraviolet emission of the solar corona and diagnose, with unprecedented temporal coverage and space resolution the structure and dynamics of the full corona in the range from 1.2 to 3.0 (1.6 to 4.1) solar radii (R⊙, measured from Sun centre) at minimum (maximum) perihelion during the nominal mission. It can also perform spectroscopic observations of the solar disk and out to 1.4 R⊙ within the 50-150 nm spectral region, and of the geo-effective coronal region 1.7-2.7 R⊙ within the 30-125 nm spectral band.

Calibration of the EKPol K-corona imaging polarimeter

The Dual Rotating Retarder Polarimeter technique has been used for the calibration of the EKPol polarimeter, which is a K-corona imaging instrument based on a Liquid Crystal Variable Retarder (LCVR), and designed to measure the linear polarized radiation coming from the solar corona during total solar eclipses. We put a major emphasis on the EKPol properties at different wavelengths and temperature. In particular, the chromatic dependence of the LCVR rotation prevents from using large band observations, owing to the loss of contrast in the measured modulation curves. This study is also intended as a basis for the design of achromatic LCVRs.

The Ultraviolet and Visible‐light Coronagraph of the HERSCHEL experiment

The Herschel (HElium Resonant Scattering in the Corona and HELiosphere) experiment, to be flown on a sounding rocket, will investigate the helium coronal abundance and the solar wind acceleration from a range of solar source structures by obtaining the first simultaneous observations of the electron, proton and helium solar corona. The HERSCHEL payload consists of the EUV Imaging Telescope (EIT), that resembles the SOHO/EIT instrument, and the Ultraviolet and Visible Coronagraph (UVC).UVC is an imaging coronagraph that will image the solar corona from 1.4 to 4 solar radii in the EUV lines of HI 121.6 nm and the HeII 30.4 nm and in the visible broadband polarized brightness. The UVC coronagraph is externally occulted with a novel design as far as the stray light rejection is concerned. Therefore, HERSCHEL will also establish proof‐of‐principle for the Ultraviolet Coronagraph, which is in the ESA Solar Orbiter Mission baseline.The scientific objectives of the experiment will be discussed, togetherwith a des...

Optical design of a high-spatial-resolution extreme-ultraviolet spectroheliograph for the transition region

A spectroheliograph dedicated to the observation of the solar disk in the extreme-ultraviolet OV spectral line at 62.97 nm is described. As demonstrated in the Skylab SO-82A spectroheliograph [Appl. Opt. 16, 870 (1977)], this line is uniquely suited to characterize solar plasma in the important 250, 000 K temperature regime. No multilayer coating or suitable filter is yet available to select this wavelength, so an optical design based on a double spectrograph with a spatial filter to remove the unwanted radiation has been developed. Analysis of the optical design shows that this instrument can obtain a 1 arcsec spatial resolution (two pixels) with a relatively high image-acquisition cadence. A preliminary tolerance analysis has been performed. A simple method of instrument alignment in visible light is also described.

Formation flying metrology system for the ESA-PROBA3 mission: the Shadow Positioning Sensors (SPS)

PROBA3 is the first high precision formation flying (FF) mission under responsibility of the European Space Agency (ESA). It is a technology mission devoted to in-orbit demonstration of the FF techniques, with two satellites kept at an average inter-satellite distance of 144m. The guiding scientific rationale is to realize a diluted coronagraph with the telescope (ASPIICS) on one satellite and the external occulter on the other satellite to observe the inner Solar corona at high spatial and temporal resolution, down to 1.08R⊙. The two spacecraft will be orbiting in a high eccentricity geocentric trajectory with perigee at 600km and the apogee at 60000Km and with an orbital period of 19hrs. The FF acquisition and operations will last about 6 hrs around the apogee and different metrology systems will be used for realizing and controlling the FF. The alignment active most critical sub-system is the Shadow Positioning Sensors (SPS), a series of Si-PM (Silicon Photomultiplier) disposed around the ASPIICS telescopes entrance aperture and measuring the proper positioning of the penumbra generated by the occulter at the center of the coronagraph’s optical reference frame. The FF alignment measurement accuracies required to the SPS are: 500μm for lateral movements and 50mm for longitudinal movements. This paper gives an overview of the opto-mechanical and electronic design and of the software algorithm for the FF intersatellite positioning. The expected performance of the SPS metrology system are reported.